Pressurized water nuclear reactors generally comprise a vessel enclosing the reactor core which is immersed in the pressurized water for cooling the reactor.
The vessel of generally the reactor, of cylindrical shape, comprises a head in the form of a spherical cap which can be added onto its upper part so that the head presents a convex outer upper surface. The head is pierced by apertures at the level each of which is secured by welding in vertical position, a tubular penetration piece constituting an adaptor ensuring the passage of an extension of a cluster for controlling the reactivity of the core or the passage of a measurement means, such as a thermocouple column, to the interior of the core.
Each of the tubular penetration pieces is engaged in an aperture passing through the head of the vessel and welded onto the concave lower surface of the head by an annular weld.
The adaptors intended to receive an extension of a cluster for controlling the reactivity of the core have an upper part projecting above the head of the vessel onto which is secured a control mechanism serving to move the extension securely attached to the cluster termed control rod.
Inspections carried out on nuclear reactors in service have shown that some of the penetration adaptors of the head of the vessel are liable to have cracks, after a certain time of utilization of the head, in particular in the weld zone of the adaptors, in the vicinity of the concave lower surface of the head. These cracks are liable to develop until they completely pass through the wall of the adaptor. In this case, limited but troublesome leakage of primary fluid constituted by the pressurized water may occur, to the outside of the head, through the crack and the space included between the adaptor and the penetration aperture of the head. The pressurized water constituting the leakage liquid, which is at very high temperature and which finds itself depressurized, evaporates, with the result that a release of steam occurs above the head. The steam contains some radioactive fission products which are liable to be found in the atmosphere above the vessel of the nuclear reactor. It is necessary to detect very rapidly the presence, in a penetration adaptor of the head of the vessel, of a crack right through it leading to leakage of primary fluid, in order to carry out a repair after shutting down the nuclear reactor and in order to prevent the release of gaseous radioactive substances into the confinement enclosure of the nuclear reactor.
The use of an envelope of heat-insulating material termed casing, which covers the convex upper surface of the head of the vessel in the penetration zone of the adaptors is known. This heat-insulating envelope serves to reduce the thermal flux emanating from the vessel of the reactor, in the direction of the control mechanisms secured to the ends of the adaptors.
The casing generally comprises a plane upper wall which comprises apertures permitting assembly of the casing around the adaptors comprising the control mechanisms, at the time of its installation. In contrast, the casing comes to rest, generally through the intermediary of a cylindrical lateral wall, on a support of the skirt for ventilation of the head of the vessel, the whole being assembled in non-leaktight manner.
As a result, the heat-insulating envelope does not ensure the confinement of the leakage gases which might be released above the head of the vessel; such leakage gases would on the contrary be carried away by the air for cooling the cluster control mechanisms, which circulates around the casing.
It is therefore not possible to carry out a detection of the possible presence of radioactive leakage gases above the head, inside the heat-insulating envelope.
In a copending application, applicants have proposed the use of a leaktight heat-insulating enclosure for confining and covering the head of the vessel of a pressurized water nuclear reactor.
The confinement enclosure which is constructed so as to be as leaktight as possible to the leakage gases which may emanate from the vessel is fixed onto the vessel head and around the adaptors, so as to prevent any substantial escape of gases. This leaktight enclosure delimits with the upper surface of the vessel a leaktight chamber in which the leakage gases emanating from the vessel of the reactor will be trapped. In order to detect the presence of radioactive gases revealing a leak at the level of an adaptor, there is associated with the confinement enclosure a device for sampling and possibly for injection of gases which is connected to a detection installation.
Preferably, such a leaktight enclosure is constructed in modular form by assembling wall members such as panels which comprise joining means fitted with leaktight seals.
In order to ensure effective detection of the presence of adaptors showing a leak, it is necessary to provide sampling means distributed in a satisfactory manner on the wall of the enclosure.
Means for constructing an enclosure which, during the construction of the enclosure provide a means for effective sampling of gases inside this enclosure, are unknown in the art.
This problem and the solution provided by the invention have a much more general scope than an application limited to the detection of leakage through the head of the vessel of a pressurized water nuclear reactor.
In fact, in the case of an enclosure or pipeline, enclosing a fluid at high temperature and possibly at high pressure, it may be advantageous to provide a heat-insulating enclosure surrounding all or part of the outer surface of the receptacle or of the pipeline, which can be constructed in modular form and which makes it possible to sample gases from or to inject gases into a chamber delimited by the wall of the enclosure and by the outer surface of the receptacle or of the pipeline.